AlGaN/GaN Nanowires: from Electron Transport to RF Applications

Gallium Nitride (GaN) and all III-Nitride compounds have revolutionized the world with the development of the blue light emitting diode (LED). In addition, GaN-based epi-structures, such as AlGaN/GaN, enable the fabrication of high electron mobility transistors (HEMTs) for several applications handling high power. In the radio frequency (RF) domain, power amplifiers (PAs) based on GaN HEMTs are currently used for base stations, radars and, in the future, will replace low power amplifiers, currently made with silicon, ensuring much smaller footprint and reduced cost. Despite the presence of several commercial GaN HEMT-based devices, performances, especially in frequency, are quite far from the predicted theoretical limits. A lot of research focuses in using etched AlGaN/GaN HEMT nanowires (NWs) as channel for ultrascaled metal semiconductor field-effect transistors (MESFETs) to reduce short channel effects, arising when shrinking the gate length to increase the operating frequency, and linearity issues. In this thesis, we perform a transverse study of AlGaN/GaN NWs starting from the investigation of electron transport and ending with the proposal of new device architectures for improved device performance.

Electron transport is studied in mesoscopic devices based on GaN HEMT NWs. Ballistic transport is observed even at room temperature and at large bias, thanks to the high mobility of electrons and large optical phonon energy. Quenched Hall effect and quantum physics phenomena due to phase coherence are observed at cryogenic temperature under a strong magnetic field. Finally, the sidewall depletion width, an important parameter for designing devices based on NWs, of AlGaN/GaN is extracted, resulting in one of the smallest values (19.5 nm) among all III-V HEMTs.

Due to the increased parasitic capacitance in NW-based MESFETs, we also explored more exotic gating techniques consisting of side and in-plane gates (IPGs). IPGFETs and metal IPGFETs (M-IPGFETs) are fabricated, characterized and simulated. A record current density of 1.4 A/mm and transconductance of 665 mS/mm are measured, together with a breakdown voltage larger than 300 V. Simulations of the device capacitance yield values as low as few aF, resulting in possible cut-off frequencies up to 0.89 THz. Such value, together with the large breakdown voltage, push GaN IPGFETs beyond the theoretical limit of GaN for RF applications and paves the way for future device architectures.

NWs are also used to noticeably improve performance of zero-bias RF detectors. We developed for the first time a field-effect rectifier (FERs) where the channel is constituted by an array of GaN-HEMT etched NWs. The much better gate control over NWs resulted in a current curvature (30.1 1/V) close to the theoretical limit of Schottky diodes (38.7 1/V). The cut-off frequency of these devices is up to 140 GHz, which, together with the large measured responsivity (3000 V/W), make NW-FERs very promising for stand-alone applications as well as for integration in GaN Monolithic Microwave Integrated Circuits (MMICs) for improving overall performance and add new applications.